The present invention relates to automatic transmissions, and more particularly to a control system for an eight-speed automatic transmission.
A typical automatic transmission in a motor vehicle has two or three planetary gear sets, one of which receives a torque input from an engine, another one of which is coupled to a drive shaft for providing a torque output. During operation, a set of frictional units couple the torque input from the engine to one or more members of the planetary gear sets. Simultaneously, another set of frictional units holds stationary one or more members of the planetary gear sets. These frictional units provide different ratios of input-to-output torque to the vehicle.
Automatic transmissions are typically controlled by a hydraulic control system. These hydraulic control systems are used to engage and disengage the frictional units of the transmission according to the ratio of torque needed. A typical hydraulic control system is disclosed in U.S. Pat. No. 6,159,124 to Redinger et al., herein incorporated by reference. The typical hydraulic control system is composed of various valves that direct and regulate hydraulic pressure to the frictional units via various fluid passages.
A typical automatic transmission has four gear speeds with at most seven frictional units providing four forward torque ratios and a reverse. Transmissions achieving more torque ratios have been burdened by the need for additional valves to operate more frictional units. These valves are expensive and can decrease the reliability of the transmission. Therefore, it is an object of the present invention to provide an eight-speed automatic transmission providing eight forward torque ratios that requires few additional valves in the hydraulic control system as compared to existing four speed automatic transmissions. It is a further object of the present invention to provide this transmission with an electronically modulated converter clutch when the transmission is in a park/neutral position for proper operation of an auxiliary Power Take-Off Unit (PTO) mounted to and driven by the transmission.
The eight-speed automatic transmission of the present invention is controlled by a hydraulic control system. The hydraulic control system has a pressurized fluid source and a manual valve selectively movable to a reverse, drive, manual/low, and park/neutral positions. The manual valve communicates with the pressurized fluid source by a first fluid passage having a multiselect solenoid. A second fluid passage communicates with the clutches, brakes, and manual valve, and has a plurality of pulse-width solenoids disposed between the clutches and the manual valve and between the brakes and the manual valve.
The hydraulic control system has a torque converter in fluid communication with the pressurized fluid source through a third fluid passage. The torque converter is electronically modulated that is activated when a torque converter pressure valve opens the third fluid passage to the torque converter. A fourth fluid passage communicates with a switching valve and with the manual valve. A fifth fluid passage communicates with the switching valve and the pressurized fluid source and has a converter clutch solenoid disposed between the switching valve and the pressurized fluid source. A sixth fluid passage communicates from the switching valve to the torque control pressure control valve.
When the manual valve is in the park/neutral position and the multiselect solenoid is in an open position, fluid from the pressurized fluid source flows through the first fluid passage to the manual valve, through the manual valve, through the fourth fluid passage to the switching valve, and through the sixth fluid passage to the torque converter pressure valve, thereby opening the third fluid passage to the torque converter and activating the electronically modulated converter clutch.
When the manual valve is in the drive and manual/low positions, fluid from the pressurized fluid source flows through the first fluid passage to the manual valve, through the fifth fluid passage to the switching valve when the converter clutch solenoid is in an open position, through the switching valve, through the sixth fluid passage to the torque converter pressure valve, thereby opening the third fluid passage to the torque converter and activating the electronically modulated converter clutch.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.